1. Field of the Invention
The present invention relates to a spread illuminating apparatus suitable for use as a backlight unit for a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display (LCD) device is extensively used as a display device for electronic appliances, and the like, and recently is increasingly replacing a cathode ray tube (CRT) also in a relatively large display device for such as a personal computer and a television set. For such a large LCD device, a direct light type backlight has been conventionally predominant which is structured such that a plurality of cold cathode lamps (hereinafter referred to as CCFLs as appropriate) as light sources are disposed immediately beneath an LCD panel, and in which light is condensed by an optical sheet and light intensity is uniformed by a diffuser sheet (refer to, for example, Japanese Patent No. 3653274).
In many of such direct light type backlights with CCFLs, all CCFLs are driven so as to be kept lighted at all times during a period when a picture image is displayed on the LCD panel, and an illuminating light basically has a predetermined view angle characteristic (that is to say, a characteristic inherent and specific to a backlight) such that brightness usually exhibits its peak when viewed from the front although means to improve the characteristic have been disclosed (refer to, for example, Japanese Patent Application Laid-Open No. 2005-221619).
In the meantime, recently, various approaches as described below are attempted for improving picture image quality and enhancing performance in an LCD device, and accordingly a backlight is becoming required to offer less conventional features.
For example, as a means of solving the visibility issue of movie display on an LCD panel, what is called a “pseudo impulse driving method” is established, in which a black display period is forcibly inserted between every two adjacent frame periods of a picture image display, whereby the pixel brightness response of the LCD panel is brought close to the impulse response.
In the pseudo impulse driving method, the black display period is proposedly produced by turning off the backlight, in which case the backlight is required to be capable of turning on and off rapidly. The black display period is also proposedly produced by inputting a black write signal in a picture image signal per frame period, in which case the backlight is preferably turned on and off rapidly in synchronization with the black write signal in order to achieve an efficient pseudo pulse impulse driving.
Especially, in case of an LCD panel with a rapid response, a method is proposed in which a black write signal is inserted in a picture image signal per scanning line at a predetermined timing, whereby a black region is displayed at an area of a screen composed of a plurality of successive scanning lines, and at the same time the position of the black region displayed is shifted in synchronization with the scanning of the picture image (hereinafter this method is referred to as “black insertion driving method” as appropriate). In this case, it is required that only an area of the entire luminous region of the backlight located immediately beneath the black region be selectively turned off in synchronization with the shifting of the black region.
Also, what is called an “area control method” is proposed, in which the lighting condition of the backlight is partly controlled for enhancing the contrast of a picture image and reducing the power consumption. In this method, the backlight is driven such that when a black region which does not need illumination is present in a picture image as a display tone factor, the area of the luminous region located immediately beneath the black region is selectively turned off.
Unlike the above-described black region (black display period) forcibly inserted for improving the movie display performance, the black region working as a display tone factor for picture image is caused to vary in shape and size on the screen. Consequently, in order to achieve an effective area control, the areas of the luminous region of the backlight, which are adapted to turn on and off independently of one another, must be configured sufficiently minute and precise.
As enhancement of performance, recently, a multiple view LCD device, such as a dual view LCD television, is proposed which enables display of multiple different picture images on one screen, such that each of lights which are emitted from a backlight and transmit pixels of an LCD panel is controlled so as not to reach an area beyond a given space region corresponding to the pixel the each light transmits, whereby respective display directions of a plurality of different picture images are separated from one another.
A backlight for such a multiple view LCD device is required to provide a view angle characteristic suitable for such a display mode that a plurality of display directions are present in one same screen. By forming a plurality of picture images into one same picture image, this type of LCD device also enables a display mode where a single picture image is viewed from the front like a normal LCD device, and so the backlight is desired to have a view angle characteristic adapted to flexibly respond to the switching of image display mode.
Conventional backlights, however, have the following difficulties in keeping up with the aforementioned improvement of image quality and enhancement of performance.
In a direct light type backlight with CCFLs, since a CCFL has a low response speed, and since the number of CCFLs provided in a backlight is usually restricted, commonly it is difficult to achieve a backlight to suitably match a pseudo impulse driving (especially, the above-described black insertion driving method).
Also, in a direct light type backlight with CCFLs, the luminous region cannot be divided into individual areas in the direction orthogonal to the length of the CCFLs because of its structure, and therefore the areas which are allowed to be area-controlled are forced to be very inadequately sized and shaped.
Meanwhile, recently, a direct light type backlight is proposed which employs, as light sources, a number of red (R), green (G) and blue (B) light emitting diodes (LEDs), in place of the CCFLs, arranged immediately beneath an LCD panel, wherein a white light is produced by mixing lights from these LEDs. Such a backlight may possibly evade the above-described problems with respect to the pseudo impulse driving and the area control.
However, the photoelectric conversion efficiency is low in achieving a white light by mixing lights from red, green and blue LEDs, thus raising problems with power consumption, heat value from light sources, cost, and the like for achieving a white light with an adequate brightness.
Further, the conventional direct light type backlight, regardless of using CCFLs or LEDs as light sources, has, as described above, a predetermined view angle characteristic such that the brightness is usually at its peak when viewed from the front. If such a backlight is used in a multiple view LCD device which has a display mode that multiple different picture images are displayed in a plurality of different directions, then the view angle characteristic of the backlight does not fit the display mode of the multiple view LCD device described above, and consequently the brightness characteristic proper to the backlight cannot be fully utilized thus inviting brightness deterioration of the LCD device. And, this brightness deterioration in the conventional backlight must usually be compensated for by increasing electric power inputted to the backlight, thus increasing power consumption.